Many substances are known to stimulate EC production of PGI2 and t-PA. Additionally, it has been reported that fibrin can Be formed on the EC surface. In this study, the possibility that fibrin generated on the surface of cells can stimulate production of PGI2 and t-PA was examined. Human umbilical vein ECs were incubated for various time intervals with citrated human plasma clotted on the cells by the addition of CaCl2 . Control dishes contained plasma without Ca++ or serum. Time-dependent generation of PGI2 and t-PA was seen over 22-24 hours. Maximal production of PGI2 occurred when fibrin on the cells was formed from 10 to 50% plasma, with serum comprising the remainder of the incubation volume, while maximal t-PA production occurred with clots formed from 100% plasma. Fibrin I formed by addition of batroboxin to citrated plasma stimulated less synthesis of t-PA than did fibrin formed by thrombin action, and it did not stimulate PGI2 production. Thrombin clots were significantly more adherent to the cells than were batroboxin clots. PG^ synthesis induced by fibrin was fully inhibited by indomethacin, approximately 50% inhibited by actinomycin D and cycloheximide, and 20% inhibited by trifluoperazine, but was unaffected by cytochalasin D and vinblastine. Stimulation of t-PA synthesis by fibrin was unaffected by indomethacin, completely inhibited by actinomycin D and cycloheximide, and 60%, 80% and 40% blocked by cytochalasin D, vinblastine, and trifluoperazine, respectively. Thus, thrombin-induced fibrin clots stimulated PGI2 synthesis, and both thrombin and batroboxin clots stimulated t-PA synthesis. Protein and RNA synthesis were essential to stimulation of t-PA synthesis but inhibition of these processes only partially inhibited stimulation of PGI2 synthesis. Integrity of the cytoskeleton was necessary for full stimulation of t-PA synthesis, but not for stimulation of PGI2 synthesis. Thus the mechanisms of stimulation of these two cellular products were different. Increased PGI2 production could serve to limit further fibrin formation by preventing platelets from contributing to the coagulation process and increased t-PA could stimulate lysis of existing fibrin.
STIMULATION BY INSULIN OF RNA SYNTHESIS IN CHICK FIBROBLASTS